The weather has yet to marry..
The weather in Venice varies from summer to winter, from fall to spring, from one year to the next, and from one day to the next.
Taking into account that, as a rule the Venice weather follows nevertheless its own trend.
Basically, as a rule of thumb, I can say this:
The Venice climate is tempered, with some mediterranean influence.
It means that the Veneto's capital is never too cold, the temperatures in winter rarely goes under 0°C (32°F), thanks to the mitigating influence of the sea.
In summer, the high can reach about 35°C.
Being a maritime town inside a lagoon, the humidity can often be high.
But...there's a saying in Italy which goes: the weather has yet to marry.. meaning that it's someway unpredictable and it does whatever it wants...and the weather in Venice is no different!
Having said that, Venice is located in the Italy's northeast part. That means that you can not expect the mild climate of Sicily all year long...but neither the northern european one!